1. Field of the Invention
The present invention relates to copying apparatus and particularly to a color copying apparatus for forming a color copy image by superposition of toner images of plural colors.
According to the present invention, it is possible to reduce a size of a rotating unit for superposition of toner images and to increase the number of copies per unit time while maintaining a good precision of superposition of the toner images.
2. Description of the Related Art
An analog type color copying apparatus using a single light source for scanning an original and for exposure to form a latent image on a photoreceptor forms a color copy image corresponding to the original by scanning the same original for a plurality of times, forming toner images of different colors in the respective scanning operations and superposing the toner images of the different colors.
In order to form a toner image in such apparatus, scanning light reflected from an original according to the color of the toner is applied to the photoreceptor by means of a color filter for the corresponding toner color, whereby a latent image of a separated color of the original is formed on the photoreceptor.
In general, toners of three primary colors of cyan (C), magenta (M), and yellow (Y) are used for formation of a color copy image and color separation of scanning light is made by using color filters of red (R), green (G), and blue (B). In cases of forming a black copy image by using a black (BK) toner or forming a monochromatic copy image of a single composite color obtained by combination of the above-mentioned primary color toners, the photoreceptor is exposed to light through a neutral density (ND) filter which regulates an exposure amount.
A conventional color copying apparatus in which a photoreceptor is exposed to scanning light of an original is disclosed for example in Japanese Patent Publication No. 59-37505. Such a conventional color copying apparatus includes a rotating position sensor for detecting a rotating position of a transfer rotating unit driven corresponding to scanning and controls movement of a scanner in response to a rotating position signal generated by the rotating position sensor for each rotation of the transfer rotating unit.
Thus, scanning is started in synchronization with the rotating position signal generated with a prescribed cycle, whereby timing for superposition of respective images on the rotating unit is controlled.
In the case of forming a color copy image, three or four scanning operations are consecutively performed with respect to the same original to form toner images of respective colors.
However, in such a conventional color copying apparatus, scanning is started each time the above-mentioned rotating position signal is generated and if scanning operations are to be performed consecutively, it is necessary to terminate forward and returning movements of the scanner in a cycle of generation of the rotating position signal, that is, in a period of one rotation of the rotating unit. In other words, the cycle of generation of the rotating position signal needs to be longer than the time required for the forward and returning movements of the scanner. In consequence, it is necessary to provide a large-size rotating unit having a large outer surface.
In addition, the time required for reciprocating movements of a scanner differs dependent on a size of an original, a copying magnification, forward or backward scanning in a book division mode, etc. Consequently, if the cycle of generation of the rotating position signal is set corresponding to the copy mode requiring the maximum time for reciprocating movements of the scanner among various copy modes, there is waiting time in a copy mode having a shorter time for reciprocating movements until the next rotating position signal is generated after an end of forward movement and an index or a value indicating the number of copies of an image formed per unit time is lowered.
Further, in the above-mentioned color copying apparatus, an exposure lamp may be turned on at the time of generation of the rotating position signal and counting operation may be performed to measure prescribed time required until a light amount of the exposure lamp attains a prescribed value and becomes stable. In such a case, at the end of the counting operation, forward movement of a scanner (i.e., scanning of the original) is started. Then, at the end of the time of the forward movement of the scanner defined by the size of the original, the copying magnification or the like, the exposure lamp is turned off and the scanner returns.
In such a control of the conventional color copying apparatus, scanning is started after the prescribed time required for the exposure lamp to be stable after the generation of the rotating position signal and, consequently, it sometimes happens that the scanning start timing slightly changes with respect to the timing of generation of the rotating position signal due to an error of the timer counting the prescribed time, or the like.
Thus, the conventional color copying apparatus may involve a disadvantage that a small deviation occurs in superposition of respective toner images, making it difficult to obtain a high-quality color copy image.